The present invention relates to an apparatus for measuring the wavelength, optical power, and optical signal-to-noise ratio (OSNR) of each optical signal in wavelength-division multiplexing (WDM) optical communication using optical reference signals for reference wavelengths and an optical tunable band-pass filter.
WDM technologies allow many optical signals with different wavelengths to travel together along a single fiber and increase the transmission capacity. To use such WDM technologies in communication, the wavelength, optical power, and OSNR of each optical signal should be measured for the communication administration.
FIG. 1 is a screen image of the optical spectrum illustrating the wavelengths, optical powers, and OSNRs of four optical signals when we measure optical signals a, b, c, and d by a conventional optical spectrum analyzer. The conventional optical spectrum analyzer has a rotating diffraction grating and a fixed photo-diode to measure the wavelength, optical power, and OSNR. In conventional methods, the optical power and OSNR of each optical signal is measured by using the photo-diode in the optical spectrum analyzer. In addition, the OSNR of optical signal is defined as the ratio of its optical power to its noise power.
Although such an optical spectrum analyzer has advantages of wide range and precision measurement, it is bulky and mechanically unstable.
To compensate these drawbacks, three methods were proposed.
FIG. 2 is a diagram illustrating an apparatus using a fixed diffraction grating 206 and a separate photo-diode array 205 for the measurement. This method was disclosed by U.S. Pat. No. 5,796,479, xe2x80x9cSignal Monitoring apparatus for Wavelength Division Multiplexed Optical Communicationxe2x80x9d, D. Derickson, R. L. Jungerman.
Wavelength-division-multiplexed optical signals are supplied from the optical fiber 201 and collimated by the lens 202. The halves of the collimated optical signals go through the half mirror 203 and then are diffracted by the fixed diffraction grating 206. The optical signals diffracted by the diffraction grating 206 go through the polarization compensator 207 to reduce the polarization dependence of the measurement. Then, all of the optical signals are reflected upon the flat mirror 208 and go through the polarization compensator 207 again. The diffraction grating 206 diffracts the optical signals again. The halves of the diffracted wavelength-division optical signals are reflected right angle by the half mirror 203 and focused to the photo-diode array 205 by the lens 204.
The photo-diode array 205 consists of separated photo-diodes. The separated photo-diodes are a pair of the photo-diodes that are slightly separated. Each of them is spatially located at the position where the optical signal with the wavelength of ITU-T standard grid is irradiated.
Accordingly, if the wavelength of the irradiated optical signal is the same as ITU-T standard grid, the electric outputs of the separated photo-diodes are equal. However, if the wavelength of the irradiated optical signal is not the same as ITU-T standard grid, the electric outputs of the separated photo-diodes are not equal. Consequently, the wavelength of each of the wavelength-division-multiplexed signals can be estimated on the basis of ratio of the electric outputs.
The optical power of each optical signal is measured by using the total power of the separated photo-diodes. In addition, the optical power measured by the photo-diode of DN, located between the separated photo-diodes as shown in FIG. 2, is used to approximate its noise power. However, this conventional method is disadvantageous in that the optical fiber 201, the diffraction grating 206, and the photo-diode array 205 must be exactly aligned in free space.
FIG. 3A is a diagram illustrating an apparatus with a fixed diffraction grating and a photo-diode array for measuring the wavelength, optical power, and OSNR of each optical signal in WDM optical communication according to conventional methods. This method was disclosed by xe2x80x9cA high-performance optical spectrum monitor with high-speed measuring time for WDMxe2x80x9d, K. Otsuka et al. at 1997 European Conference on Optical Communication.
Wavelength-division-multiplexed signals supplied by optical fiber 301 are polarization-compensated at polarization compensator 302. The compensated signals are collimated by a lens 303 and diffracted by the fixed diffraction grating 304. The diffracted signals are focused by a lens 305 and flat mirror 307 and irradiated to the photo-diode array 308.
In this apparatus, the wavelength, optical power, and OSNR of each of the wavelength-division-multiplexed signals are obtained on the basis of Gaussian approximation using the result of spatially discrete measurement. FIG. 3B shows an example of Gaussian approximation, which is based on discrete values measured by the photo-diode in FIG. 3A.
Same process is applied to other optical signals of different wavelengths.
However, as illustrated at FIG. 3a, the optical fiber 301, the diffraction grating 304, and the photo-diode array 308 are spatially separated and therefore complicated free-space alignment among those devices is required for accurate measurement. Also, Gaussian approximation is an overhead.
FIG. 4 is a diagram illustrating an apparatus with a blazed bragg grating for measuring the wavelength, optical power, and OSNR of each optical signal in WDM optical communication according to the conventional methods. This method was disclosed by xe2x80x9cHigh Resolution Fiber Grating Optical Network Monitorxe2x80x9d, Chris Koeppen et al. at National Fiber Optic Engineers Conference 1998.
A blazed bragg grating 401 is inscribed on optical fiber 402. A part of wavelength-division-multiplexed signals are reflected on the blazed bragg grating 401 and irradiated to the photo-diode array 403 through the glass block 404. The methods for measuring the wavelength, optical power, and OSNR of each optical signal are the same as those of the apparatus shown in FIG. 3A.
However, the photo-diode array and the blazed bragg grating are spatially separated and therefore complicated free-space alignment among those devices is required for accurate measurement.
An apparatus for measuring the wavelength and optical power and optical signal-to-noise ratio of each optical signal in WDM optical communication is provided.
The apparatus includes the following means. The splitting means splits a part of wavelength-division-multiplexed optical signals. The amplifying means amplifies said wavelength-division-multiplexed optical signals and generates spontaneous emission light simultaneously. The reflection means reflects a predetermined section of said spontaneous emission light and generates an optical reference signal for reference wavelength. The combining means combines said optical reference signal with said wavelength-division-multiplexed signals and generates the combined light. The wavelength-division-multiplexed signals are split by said splitting means. The filtering means filters said combined light at fixed temperature and generates the same waveform as the optical spectrum of said combined light in time domain. The converting means converts said waveform into an electric signal. And, the signal-processing means measures said wavelength, said optical power, and said optical signal-to-noise ratio of each of said wavelength-division-multiplexed optical signals by employing said electric signal.
Preferably, the apparatus further includes signal-generating means for generating control signal. The control signal controls said filtering means and said signal-processing means.
Preferably, the apparatus further includes optically isolating means for passing the optical signal passing through said reflecting means in uni-direction.
An apparatus for measuring the wavelength and optical power and OSNR of each optical signal in WDM optical communication according to another embodiment of the present invention is provided.
The apparatus includes the following means. The splitting means splits a part of wavelength-division-multiplexed optical signals. A spontaneous emission light source generates spontaneous emission light. The reflecting means reflects a predetermined section of said spontaneous emission light and generates an optical reference signal for reference wavelength. The combining means combines said optical reference signal with wavelength-division-multiplexed signals and generates the combined light. The wavelength-division-multiplexed signals are split by said splitting means. The filtering means filters said combined light at fixed temperature and generates the same waveform as the optical spectrum of said combined light in time domain. The converting means converts said waveform into an electric signal. And, The signal-processing means measures said wavelength, said optical power, and said optical signal-to-noise ratio of each of said wavelength-division-multiplexed optical signals by employing said electric signal.
Preferably, the apparatus further includes signal-generating means for generating control signal. The control signal controls said filtering means and said signal-processing means.
Preferably, the apparatus further includes optically terminating means for terminating spontaneous emission light without any reflection of said spontaneous emission light passing through said means for reflecting.
More preferably, the spontaneous emission light source is a light-emitting diode (LED).
More preferably, the optical reference signal is discerned by applying a driving current into said LED, which is composed of a constant current and an alternating current with specific frequency.
An apparatus for measuring the wavelength and optical power and OSNR of each optical signal in WDM optical communication according to another embodiment of the present invention is provided.
The apparatus includes the following means. The amplifying means amplifies wavelength-division-multiplexed optical signals and generates spontaneous emission light simultaneously. The reflecting means reflects a predetermined portion of spontaneous emission light and generates an optical reference signal for reference wavelength. The combining means combines said optical reference signal with said wavelength-division-multiplexed signals and generates the combined light. The wavelength-division-multiplexed signals pass through said reflecting means. The filtering means filters said combined light at fixed temperature and generates the same waveform as the optical spectrum of said combined light in time domain. The converting means converts said waveform into an electric signal. And, signal-processing means measures said wavelength, said optical power, and said OSNR of each of said wavelength-division-multiplexed optical signals by employing said electric signal.
Preferably, the apparatus further includes signal-generating means for generating control signal. The control signal controls said filtering means and said signal-processing means.
Preferably, the amplifying means is one of an erbium-doped fiber amplifier and a semiconductor optical amplifier.
Preferably, the reflecting means is one of a fiber bragg grating and an integrated optical device including grating.
Preferably, the filtering means is one of a tunable Fabry-Perot filter, an integrated optical device including grating, and a multi-layer thin film device.